This invention relates to techniques for non-invasively detecting the concentration of analytes in the blood of living animals, and in particular to the use of continuous spectrum infrared spectroscopic techniques for the non-invasive detection of glucose concentrations in the blood of humans.
In the diagnosis and treatment of various conditions, it is important to measure the concentration of various constituents in the blood. For example, in the treatment of diabetes, the concentration of glucose in the blood must be measured on a periodic basis. For persons experiencing insulin-dependent or Type I diabetes, it is often necessary or desirable to measure blood glucose concentrations several times each day. Obtaining accurate readings of cholesterol concentrations is important in the prevention of coronary artery disease. The measurement of the concentration of other blood analytes, such as bilirubin and alcohol, is also important for various diagnostic purposes.
The accurate measurement of concentrations of such blood constituents, as it is now practiced, requires obtaining a blood sample, such as by pricking a finger. The obtaining of blood samples by invasive techniques, such as pricking the finger, is both painful and inconvenient. In the case of diabetics, the need to lance a finger several times a day to monitor glucose levels result in a buildup of scar tissue. Indeed, many diabetics are believed not to monitor their glucose levels as frequently as recommended because of the pain and inconvenience of the invasive method. The result of such a failure to monitor glucose levels is a greater risk of experiencing the long-term health effects of diabetes. These health effects include damage to the eyes, resulting in partial and often total loss of vision, as well as other serious health problems. Millions of individuals in the United States alone suffer from diabetes. As a result, the failure of an individual afflicted with diabetes reliably to monitor their glucose levels is a significant public health problem.
In order to provide an alternative to the existing invasive blood glucose monitoring techniques, non-invasive blood glucose detection techniques have been proposed. One such technique is the non-invasive continuous spectrum infrared spectroscopic technique. One example of such a technique is given in U.S. Pat. No. 5,070,874 (Barnes, et al.) In this technique, a portion of the patient's body is non-invasively irradiated with infrared radiation across a continuous spectrum. Radiation emitted from the body part, which radiation has been either transflected or transmitted, is then detected, to obtain signals representing the intensity of radiation at numerous wavelength ranges within the continuous spectrum. The signals are then processed to obtain an absorbance spectrum. Appropriate analytical techniques are applied to the detected absorbance spectrum in order to obtain a blood glucose level. Concentrations of other blood analytes may also be measured in this manner.
No device using the non-invasive infrared technique has achieved accuracy sufficient to match that of existing invasive techniques. A significant difficulty in obtaining sufficient accuracy is a low signal-to-noise ratio. Continuous-spectrum noninvasive techniques make use of radiation in the near-infrared portion of the spectrum. However, in this portion of the spectrum, the absorption of radiation by water is very high. In addition, the concentrations of the analyte of interest in the bloodstream is typically low. As a result, the contribution of the analyte of interest to the signal intensity is only a relatively small change in the total signal intensity obtained by this technique. It has been found that detector noise is of the same order of magnitude as the change in intensity signal resulting from variations in analyte concentration. The variations in signal intensity as a result of variations in concentration of the analyte of interest are so small that, at intensities that have been used in the past, the detector's sensitivity may not be high enough to obtain sufficiently accurate readings.
A possible solution to this problem would be to increase the intensity of the radiation incident on the body part of the subject. However, an increase in the intensity of incident radiation increases the amount of energy absorbed by the body part. Increases in the energy absorbed by the body part result in greater heating of the body part the amount of heat produced. Excessive heating can cause discomfort and even burns to the subject, which obviously would be undesirable.
It is accordingly an object of this invention to provide a method for the continuous spectrum non-invasive spectroscopic detection of analytes in the bloodstream of living animals with increased signal-to-noise ratio.
Further objects and advantages of the invention will become apparent from the detailed description of a preferred embodiment which follows.